1. Field of the Invention
The present invention relates to a high-resolution solid-state NMR spectrometer used when nuclear magnetic resonance (NMR) spectra are acquired from disklike solid samples such as wafers. The invention also relates to a sample holder used in this spectrometer and to a method of solid-state NMR spectroscopy.
2. Description of the Related Art
An NMR spectrometer is an analytical instrument for detecting a signal arising from atomic nuclei having spin magnetic moments by applying a static magnetic field to the nuclei to induce the spin magnetic moments to produce a Larmor precession and irradiating the nuclei with RF waves having the same frequency as the precession to bring the nuclei into resonance.
Samples to be investigated by NMR include two types: solution samples and solid samples. Among them, many solution samples give quite sharp NMR spectra and, therefore, it is widespread to perform molecular structural analysis of chemical substances by making full use of the excellent performance of the obtained high-resolution NMR spectra.
On the other hand, in an NMR spectrum of a sample in solid phase, interactions (such as dipolar interactions) which would be nullified by rotational Brownian motion in a solution manifest themselves directly and so the spectral linewidth broadens extremely, thus obscuring chemical shift terms. Therefore, in an NMR spectrum, it is impossible to isolate the signal peaks arising from various portions of a molecule under investigation. As a result, it has been thought that solid-state NMR spectroscopy is unsuited for molecular structural analysis.
A method which overcomes this undesired phenomenon and gives rise to sharp solid-state NMR spectra was discovered by E. R. Andrew in 1958. In particular, anisotropic interactions are removed and chemical shift terms can be extracted by tilting the sample tube at an angle of about 54.7° to the direction of the static magnetic field B0 and spinning the tube at high speed. This method is known as MAS (magic angle spinning).
A solid-state NMR instrument includes a mechanism for adjusting the angle of the axis of spinning of a sample tube. The mechanism is shown in the block diagram of FIG. 1. The instrument has a probe, generally indicated by reference numeral 1. The sample tube, indicated by numeral 2, holds a sample therein and is also termed a rotor. The tube 2 is inserted in a sample spinning mechanism (stator) 3 having an air bearing and is spun at high speed using a gaseous medium such as compressed air or nitrogen gas.
A movable mechanism 4 such as a toothed wheel is used to vary the angle of the stator 3. A shaft 5 or the like is connected to the movable mechanism 4 to permit the movable mechanism 4 to be controlled from the outside. A knob 6 that is connected with the shaft 5 is accessed and manipulated by a user when the magic angle is actually adjusted.
Chemical shift anisotropy can be eliminated and the NMR spectral linewidth can be sharpened by spinning the sample tube at the magic angle of 54.7° to the static magnetic field B0. Therefore, adjustment of the magic angle has become an important technique.
In recent years, semiconductor thin-film growth technology has evolved. As a result, attempts to evaluate the physical properties of a thin-film sample on a wafer by investigating the film by solid-state NMR spectroscopy have begun. Its most fundamental method consists of scraping off the thin-film sample from the supporting basic material, loading the sample into a sample tube, and investigating the sample by an ordinary solid-state NMR spectrometer (non-patent document 1). In this method, however, the film is processed and, consequently, the reliability of the obtained data tends to be questioned.
Accordingly, a special solid-state NMR spectrometer capable of performing in-situ high-resolution solid-state NMR measurements without destroying thin-film samples such as semiconductor wafers have been proposed (patent document 1). Since this technique is important and provides a basis of the present invention, it is now summarized by referring to drawings.
FIG. 2 shows a high-resolution solid-state NMR spectrometer disclosed in JP-UM-A-62-79151, the spectrometer being adapted for measurements of wafers. The instrument has a magnet 11 that produces a static magnetic field H0 in which a sample holder 12 having a sample-holding surface tilted at an angle of about 35.3° to the magnetic field is disposed. A sample 13 shaped like a circular disk similar to the sample holder 12 is placed on the holder 12. A rotatable shaft 14 is mounted to the holder 12 and supported by an air bearing 16 to which pressurized air is sent from a compressor 15. Plural grooves 17 are formed in the outer surface of the shaft 14. A nozzle 19 is attached to one end of the air bearing 16 to permit a rotating force to be applied to the shaft 14 when pressurized air from another compressor 18 is blown against the grooves 17 in the shaft 14. Therefore, the sample holder 12 is spun at high speed while the tilt angle of about 35.3° to the static magnetic field is maintained.
A transmit/receive coil 21 is attached at the front end of an arm 20 and located in close proximity to the center of spinning of the sample 13 that is spun at high speed together with the holder as described previously. The arm 20 is rotatably mounted to a support base 22. A transmitter circuit 23 supplies excitation pulses to the transmit/receive coil 21. A receiver circuit 24 is used to extract a resonance signal induced in the transmit/receive coil 21. The extracted resonance signal is processed by a computer 25.
In this configuration, since the sample 13 is spun on the surface that is tilted at an angle of about 35.3° to the static magnetic field, the axis of spinning is tilted at an angle of about 54.7° to the magnetic field, i.e., set to the magic angle.
A pulse sequence (excitation pulses) is sent from the transmitter circuit 23 to the transmit/receive coil 21 on the sample surface placed in proximity to the center of spinning such that the sample is irradiated with the pulse sequence. After the irradiation, a resonance signal induced in the transmit/receive coil 21 is taken from the receiver circuit 24. Thus, an NMR measurement can be made of a portion of the sample surface lying around the center of spinning.
If the transmit/receive coil 21 is moved along the rotating sample surface, NMR measurements can be made of an annular region extending along a circle of an appropriate radius from the center of spinning.